Background: The high rates of childhood overweight and obesity in the United States have generated interest in schools as sites for monitoring body mass index (BMI) information. This study established baseline values for a 5-year longitudinal assessment of BMI of elementary school children and examined variation across the schools, because little is known about factors that affect the distribution of overweight and obesity within school districts. Methods: Height and weight measurements were collected on 2317 elementary school children in 1 school district. BMI was calculated using the Centers for Disease Control and Prevention's NutStat program. Child characteristics included gender, age, eligibility for free and reduced lunch (proxy for socioeconomic status [SES]), school, grade, and ethnicity/race. Children were grouped into 2 BMI categories, less than 85th percentile or equal to or greater than 85th percentile (overweight/obesity). Logistic regression was used to examine potential predictors of overweight/obesity. Results: Prevalence of greater than or equal to 85th percentile was 30.9%, 34.4%, 35.3%, 36.4%, 37.1%, and 44.5% for K-5, respectively. Prevalence of equal to or greater than 85th percentile was highest among Hispanic children. Ethnicity was the strongest predictor of inclusion in the equal to or greater than 85th percentile category followed by grade and free and reduced lunch eligibility. Conclusion: The data are consistent with the prevalence of overweight/obesity among American children and Hispanic children in particular. District prevalence of overweight/obesity is higher than available state statistics. Most of the BMI variation is accounted for by ethnicity, SES, and grade. The grade effect and high prevalence of overweight/obesity provide a rationale for BMI screening retention at the schools. (Contains 2 tables and 1 figure.)